Technology growth in weapon system design has produced remarkable advances in fuzing technology and capability since the Viet Nam war. Improvements in timing accuracy, target detection capabilities and target impact survivability will undoubtedly increase the effectiveness of large caliber munitions on the battlefields of the future. Unfortunately there exists one common area of fuze technology which, though better understood and more tediously analyzed than in the past, has failed to keep pace with modern weapons development--the safe and arming (S&A) device. Generally, the S&A device arms the fuze of the projectile at some required distance from the weapon to prevent premature explosion and the consequent damage to weapon and personnel.
Traditionally, the delay in arming of the fuze is accomplished through an electronic timer or by means of a mechanical device. The electronic timer is generally unsatisfactory because at the present time many weapons fire projectiles at varying muzzle velocities. Thus, a fixed time electronic timer would arm the fuze at different distances from the muzzle of the weapon, depending upon the muzzle velocity. One of the prime objectives of S&A devices is to arm the fuze at a constant required distance from the muzzle of the weapon. Mechanical S&A devices generally are limited to clockwork forms. Logistic considerations and concern during the Viet Nam conflict led to intensive investigations into alternatives to horological components to achieve arming delay. Mobilization concerns also prompted investigations into alternative methods of manufacture for pinions as a second approach to the precision component problem. While all these efforts have served to aleviate the problem somewhat, the net result has been to retain clockwork forms in S&A's simply because it is not practical to achieve meaningful time delay with ball rotors, sliders, unwinding ribbons, or mass transfer devices.
Clockwork mechanisms used in S&A's generally involve the use of runaway escapements, which when coupled to a centrifugal gear output produce a turns-counting effect in a spinning fuze. Experimentation with such devices usually results in the realization that a definite upper limit exists for arming delay for a given fuze volume. Experimentation to date seems to indicate that a realistic upper limit for runaway escapement devices in approximately a 1.3 inch diameter is somewhere between 30 and 35 turns. Longer delays may be feasible, but require either very precise control over lubrication or attendant losses in reliability. Friction becomes the main deterrent in any attempt to significantly improve arming delay in a mechanical device. Motion of a runaway escapement system is purely a matter of net torque about the pivot of the mechanical oscillator. This net torque represents the difference between applied torque from the escape wheel and pivot friction losses from the last journal bearing. Unfortunately, both of these factors are proportional to centrifugal force. Hence, a marginal design is really no better off from a reliability standpoint at a high spin than at low. Designs in which the net torque at the final stage are near zero fall prey to friction sensitivity and perform poorly in mass production. New hypervelocity weapons employing high fragmentation steel alloys could easily require substantially increased safe separation to maintain gun crew safety at present levels. A similar argument can be made for the trend toward higher spin rates in future munitions. Known wear problems and gear distortion are evident in spin regimes below 20,000 rpm with the present die-cast gears; and yet future spin rates for large caliber weapons progressively rise toward 30,000 rpm, an environment producing more than double the gear and escapement loads presently experienced.